1. Field of the Invention
The present invention relates to a reproducing apparatus for tracing tracks on a tape-like recording medium by a magnetic head and a magnetic head tracking method for use with such reproducing apparatus.
2. Description of the Related Art
In VCRs (video cassette recorders), tracking has so far been effected on a reproducing head by using a pilot signal recorded on tracks of a magnetic tape (see cited patent reference 1, for example).
FIG. 1 of the accompanying drawings is a block diagram showing an arrangement of a tracking system of a VCR according to the related art. As shown in FIG. 1, a pair of reproducing heads 1A, 1B is disposed on a rotary drum 2 with an angular extent of 180° in an opposing fashion. The reproducing heads 1A, 1B will be generally referred to as a “reproducing head 1” if necessary. The rotary drum 2 has a magnetic tape 3 wound thereon, and it allows the magnetic tape 3 to travel as it rotates. The reproducing heads 1A, 1B alternately trace the tracks on the magnetic tape 3.
FIG. 2 is a diagram showing the manner in which the reproducing head 1A reproduces pilot signals from the two tracks formed at both sides of the track to be traced at the same time it traces the tracks on the magnetic tape 3. As shown in FIG. 2, a track T1 is a track that is to be traced by the reproducing head 1A, and a track T2 is a track that is to be traced by the reproducing head 1B. Pilot signals P1 and P2 are alternately recorded on the track T2 of the magnetic tape 3.
As shown in FIG. 2, since each of the reproducing heads 1A, 1B is slightly wider than the width of the track, when the reproducing head 1A traces a certain track T1, it is unavoidable that the reproducing head 1A partly traces the two tracks T2 at both sides of a certain track T1. As is known well, so-called azimuth effect can prevent the reproducing head 1A from reproducing the signals from the track T2 at its portion in which video data and audio data are recorded. On the other hand, it is customary that the pilot signals P1, P2 are recorded by using frequencies considerably lower than those of video data and audio data such that they may become difficult to be affected by the azimuth effect (in that case, the pilot signals P1, P2 may have frequencies f1, f2 which are different from each other). Therefore, the reproducing head 1A is able to reproduce the pilot signals P1, P2 at the same time it is tracing the track T1.
Referring back to FIG. 1, reproduced RF (radio frequency) signals that have been reproduced from the magnetic tape 3 by the reproducing heads 1A, 1B are amplified by a playback amplifier (not shown) and inputted to a 2-input and 1-output switch 4a. The switch 4a is supplied with a head switching signal whose polarity goes to “L” (low) level when the reproducing head 1A scans the track and whose polarity goes to “H” (high) level when the reproducing head 1B scans the track from a suitable means such as a microprocessor (not shown) as a control signal. The switch 4a selectively outputs the reproduced RF signals reproduced by the reproducing heads 1A, 1B when the head switching control signal is held at “L” (low) level and “H” (high) level so that it may output the reproduced RF signals in the form of a time-series reproduced RF signal.
The reproduced RF signal in the form of the time-series reproduced RF signal is supplied to a video data/audio data band pass filter (video data/audio data BPF) 5 and is also supplied to a 1-input and 1-output switch 4b. Video data and audio data outputted from the band pass filter 5 are supplied to video data and audio data signal processing systems (not shown). The switch 4b is supplied with the aforementioned head switching signal as the control signal, and only when the head switching signal is held at “L” (low) level, the switch 4b is turned on to supply the reproduced RF signals to a pilot signal P1 band pass filter (pilot signal P1 BPF) 6 that is used to output the signal with the frequency f1) and a pilot signal P2 band pass filter (pilot signal P2 BPF) 7 that is used to output the signal with the frequency f2.
The pilot signals P1, P2 outputted from the band pass filters 6, 7 are respectively envelope-detected by envelope detecting circuits 8, 9 and supplied to analog-to-digital (A/D) converters 10, 11 in which they are converted in the form of analog to digital signals. Then, a subtracter 12 calculates a difference between an A/D-converted value of an envelope level of the pilot signal P1 and an A/D-converted value of an envelope level of the pilot signal P2, and an output from the subtracter 12 is supplied to a low-pass filter (LPF) 13 and thereby a tracking error signal is generated.
When the reproducing head 1 is accurately tracing a target track (when the reproducing head 1 is properly tracing a target track), the envelope level of the pilot signal P1 and that of the pilot signal P2 become identical to each other, and hence a tracking signal is canceled out. When on the other hand the reproducing head 1 is not properly tracing a target track, that is, so-called off-track occurs, the envelope level of the pilot signal P1 and that of the pilot signal P2 become different from each other, and hence the tracking error signal is not canceled out. This tracking error signal is added to a capstan control signal that a capstan control unit 14 supplies to a capstan motor 16.
Upon playback, the capstan control unit 14 detects a phase of a capstan based upon a phase difference between a reference pulse generated from a reference pulse generating unit (not shown) and a reproduced CTL pulse reproduced from the magnetic tape 3 by a CTL (control) reproducing head 15 and also detects a velocity of the capstan based upon a capstan pulse signal FG from the capstan motor 16. Then, the capstan control unit 14 generates a capstan control signal based upon the thus detected capstan phase and the thus detected capstan velocity. The tracking error signal is added to this capstan control signal, whereby the phase of the reproduced CTL pulse is controlled so as to cancel the tracking error signal out (that is, the phase of the reproduced CTL pulse is controlled so that the off-track may be corrected).
[Cited Patent Reference 1]
Japanese laid-open patent application No. 9-81991 (see paragraphs [0002] to [0009] and paragraphs [0017] to [0022] and also see FIG. 1 and FIGS. 7 to 11)
However, the above-mentioned related-art tracking system cannot avoid a defect in which tracking cannot be effected on the magnetic head rapidly in a short period of time. The reasons for this will be described below.
In the related-art tracking system, a practical value of an off-track amount (i.e. tracking displacement amount) cannot be detected from the value of the difference between the envelope level of the pilot signal P1 and the envelope level of the pilot signal P2. For example, having considered the case (a) in which the envelope levels of the pilot signals P1, P2 obtained when the reproducing head 1 properly traces the track are both held at 1V and the case (b) in which the envelope levels of the pilot signals P1, P2 obtained when the reproducing head 1 properly traces the track are both held at 2V, it is to be understood that a value of a difference between the envelope levels obtained when an off-track of a constant amount occurs in the reproducing head 1 differs in the cases (a) and (b). In this case, a value of a difference obtained in the case (b) becomes twice as large as that obtained in the case of (a).
As described above, according to the related-art tracking system, it is to be detected that “the reproducing head 1 is not properly tracing the track (that is, off-track occurs in the reproducing head 1) if the value of the difference between the envelope levels is not zero”. In addition, the direction of the off-track also can be detected from the symbol of the difference (that is, plus (+) or minus (−)). In this case, since the absolute tracking displacement amount is not known and the phase of the reproduced CTL signal should be progressively changed little by little until the difference of the envelope level is canceled out, and hence the tracking of the reproducing head 1 needs plenty of time. When the tracking of the reproducing head 1 should be completed within a constant time period such as a pre-roll time required in the VCR edit operation, if the tracking of the reproducing head 1 takes a lot of time as described above, there arise problems.
Further, since the related-art tracking system requires the band pass filters and the envelope detecting circuits of the two systems (that is, the band pass filter 6, the envelope detecting circuit 8 and the band pass filter 7, the envelope detecting circuit 9) in order to obtain the envelope levels of the pilot signals P1, P2 with the frequencies f1, f2 which are different from each other, the hardware of the above-mentioned reproducing apparatus and the hardware of such related-art tracking system become complex unavoidably.